You can keep hold of data for a good business reason such as the one you stated. A hash of the e-mail address is probably reasonable, but you might also want so

You can keep hold of data for a good business reason such as the one you stated. A hash of the e-mail address is probably reasonable, but you might also want some logic to detect users who use g-mail style addresses, i.e. "exampleuser+label@gmailcom", which allows one gmail account to have many forms of the e-mail address, in which case hashes don't work. This is another class of an example I have seen cited around from time to time - a user signs up, performs fraudulent behaviour and then demands the records of this behaviour are removed and continues to perform fraudulent behaviour. You have a good business reason to store this data so therefore the user cannot demand you remove the data. You shouldn't keep it longer than is necessary however. How long would work to deter this behaviour you describe? I'd say 30-90 days should be sufficient.